This invention relates to the field of electrophotographic recording and is more particularly concerned with an electrophotographic image carrier structure, including an electrically conductive substrate, a selenium-containing photoconductive layer which generates charge carriers and a charge carrier transporting insulating layer of polyvinyl carbazole.
Electrophotographic processes and apparatus find an ever-widening application in the copier field. These processes and apparatus utilize the property of the photoconductive material, according to which the electric resistance is changed upon illumination by means of an activating irradiation.
Subsequent to electric charging and illumination with an activating irradiation, a latent electric charge image, corresponding to the optical image, can be formed on a photoconductive layer, since at the illuminated areas the conductivity of the photoconductive layer increases to such an extent that the electric charge may be drawn away by the electrically conductive substrate--at least partially, but in any event, to a greater extent than from the non-illuminated areas--while at the non-illuminated (dark) areas the electric charge is substantially preserved. Areas retaining this charge may be made visible by a toner (image powder); the thus-produced toner image can be transferred to a record carrier, such as paper.
Organic as well as inorganic substances and, on occasion, their combination are being used as electrophotographically effective materials. Of the inorganic substances, selenium, selenium alloys and selenium compounds--particularly in an amorphous state--have been widely used. There are further known layer combinations, such as disclosed in German Laid-Open Applications (Offenlegungsschrift) Nos. 2,128,584 and 2,733,052 as well as German Pat. No. 2,444,620, wherein the selenium photoconductor is coated with light-transparent organic covering layers which are preferably polyvinyl carbazole or the like. The covering layers store the charges induced by a corona discharge and serve as charge carrier transport layers for the charge carrier which is formed in the photoconductive layer for producing the charge image on the surface of the image carrier structure.
Layer combinations made of a selenium photoconductor and a polyvinyl carbazole covering layer have a number of advantageous properties. They have a superior flexibility so that, for example, flexible bands may be used as substrates which is a significant advantage as compared to rigid plate-shaped or drum-shaped substrates. Further, the selenium photoconductive layers may be very thin and thus a substantial proportion of the very expensive selenium layers may be replaced by the significantly cheaper polyvinyl carbazole. A further advantageous property is the superior tension strength of even thin polyvinyl carbazole layers. As a result, the layers of the electrophotographic image carrier structure can be charged to high surface potentials and a high contrast potential may be achieved. Further, a polyvinyl carbazole covering layer makes feasible even a negative charging which is of advantage regarding the selection of the toner, particularly since it makes possible the use of the presently very popular single-component toners.
In contrast to all these advantages, however, the known layer combinations have the disadvantage that they have a low sensitivity, since selenium with a high proportion of arsenic, such as the particularly advantageous orthochromatic As.sub.2 Se.sub.3 compound cannot be used for the photoconductive layer. This is so because layer combinations formed of a system having As.sub.2 Se.sub.3 and polyvinyl carbazole have such a high residual potential that they cannot find practical application in electrophotography. For this reason, the known layer combinations that include polyvinyl carbazole covering layers have to be limited to the less sensitive selenium or selenium-tellurium photoconductive layers.